Question?

This program I have written has the user enter a number and it will display the fibonacci number that is associated with that number. This part works what i need to know what do I need in my code so that a user can enter 0 or a negative number so they can quit. I want it so that it will ask the user to enter a number and give the fibonacci number until a user enters a 0 or a negative number then it will end the program.